(1) Field of the Invention
The present invention relates to medical diagnostic equipment and, more particularly, to a method for manufacture of a rotary anode for X-ray tubes using high-density low-pressure plasma sprayed focal track for X-ray anodes, and the rotary anode produced.
(2) Description of Related Art
Conventional methods or processes of manufacturing a rotary anode for x-ray tubes and the anode produced are well known. For example, the U.S. Pat. No. 4,534,993 to Magendans et al. teaches a conventional method or process of manufacturing a rotary anode for x-ray tubes and the anode produced using conventional plasma spraying of coating material for the focal track onto a base unit of the anode to produce the target layer of the anode. The entire disclosure of the U.S. Pat. No. 4,534,993 to Magendans et al., issued Aug. 13, 1985 is expressly incorporated herein by this reference. Magendans et al. teaches a method of applying deposit material for the target layer onto the anode body using moderate-pressure plasma spraying techniques. However, attempts to duplicate results by those skilled in the art have not been successful, especially for larger diameter anodes.
U.S. Pat. No. 4,534,993 to Magendans et al. observed the difficulty in obtaining well-heated tungsten particles at chamber pressure less than one-half the atmospheres and, accordingly, Magendans et al. teach the use of chamber pressures between 30 kilopascals to 50 kilopascals to allow for adequate heating of tungsten particles. Of course, at these higher chamber pressures, only subsonic to sonic plasma flow velocities are created, which reduce the velocity and the momentum, and hence, the impact of the tungsten particles impinging onto the focal track to produce the target layer. As was postulated by Mark Smith, in the first Plasma-Technik Symposium, Lucerne/Switzerland, May 18-20, 1988, vol. 1, pp 77 to 85 (“Mark Smith”), this lower velocity of the particles reduce the packing density of the resulting target layer. In other words, the higher chamber pressures increased the drag forces on the tungsten particles, which lowered their velocity, which in turn, lowered their packing density in forming the focal track structure. Of course, one of the main reasons for slowing the velocity of the tungsten particles in U.S. Pat. No. 4,534,993 to Magendans et al is to allot the tungsten particles sufficient time to melt, before their impact with the base element. The allocation of sufficient time to melt the particles in the Magendans et al reference is required because of the very large differences in the tungsten particle sizes used. The particles used in the U.S. Pat. No. 4,534,993 to Magendans et al. have a grain size that range from 5 to 45 micrometers (40 micrometers difference in grain sizes), and more narrowly, defined within the range 10 to 37 micrometers (27 micrometers difference in grain sizes), which still constitutes very large mass differences. Given the large mass differences, the smaller tungsten particles (e.g., 5 micrometers) melt faster than the larger particles (e.g., 45 micrometers). Accordingly, sufficient time is required to allow the larger particle sizes to melt, and hence the need for reduction in their velocity, and the requirement for the high-pressure chamber.
The use of particles with large mass differences between particle sizes bring about another disadvantage. This large differences in particle sizes of the tungsten or tungsten alloy particles cause wide range of thermal histories and velocities between each particle, which lead to structures having multitude of defects such as re-solidified and un-melted particles entrapped between splats, which result in high levels of porosity of the focal track. Accordingly, in spite of intensive development efforts around the world in recent years, the focal track coatings using conventional plasma spraying taught by the U.S. Pat. No. 4,534,993 to Magendans et al. has not be successful. As with Mark Smith, the U.S. Pat. No. 6,132,812 to Rodhammer et al. recognized deficiencies in the teaching of the U.S. Pat. No. 4,534,993 to Magendans et al., and moved to teaching a new variant of the plasma spraying, the so-called inductive vacuum plasma spraying, which has its own set of deficiencies.
One of the deficiencies of the U.S. Pat. No. 6,132,812 to Rodhammer et al. is that it has a low feed rate of the tungsten or tungsten alloy particles, which leads to higher process or production time. The application of the U.S. Pat. No. 6,132,812 to Rodhammer et al. was limited to only 120 mm targets, and maintained the substrate temperatures by low rotation rate of the main body, which is at 10 revolutions per minute, when depositing the target layer. However, slow rotational rates for larger diameter targets will lead to conductive and irradiative losses of heat. That is, as one section of the target is heated while the target slowly rotates, the diagonally opposite section of the same away from the heat source cools, and hence, for larger targets slow rotation of the target will not function to allow even or uniform temperature for the entire target. A further disadvantage with Rodhammer et al. is the use of columnar grain structure. The columnar grain structures have a possibility of longer cracks along columnar grain boundaries.
Regrettably, both the U.S. Pat. No. 4,534,993 to Magendans et al. and the U.S. Pat. No. 6,132,812 to Rodhammer et al. lack the teaching and method for maintaining a substantially uniform temperature for the recently developed larger diameter anode bodies. Reference is made to other few, exemplary U.S. patents that also teach conventional methods or processes of manufacturing a rotary anode for x-ray tubes and the anode produced: U.S. Pat. Nos. 4,132,917; 4,224,273; 4,328,257; 5,943,389; 6,390,876; 6,487,275; and 6,584,172. Unfortunately, most prior art conventional plasma spraying methods of manufacturing a rotary anode for x-ray tubes suffer from obvious disadvantages, one non-limiting example of which is in terms of thermal management of the application of materials that produce a focal track of the anode.
In light of the current state of the art and the drawbacks to current methods of manufacturing a rotary anode for x-ray tubes mentioned above, a need exists for a method of manufacturing a rotary anode for x-ray tubes that would consider thermal management of the whole anode from conception to provide homogenously high-density focal track, and that would withstand higher energy electron bombardment then currently possible.